The main function of a steam trap is to drain condensate from a steam line in which the trap is connected, but it is desirable also to discharge air and other non-condensible gases which may collect in the steam line. For example, when steam is first turned on to cold plant there is usually a considerable volume of air which should be purged out as quickly as possible so as quickly to raise the plant to operating temperature.
In a thermodynamic steam trap there is a floating disc that serves as a valve member, mating with concentric seat rings below the disc to control communication between an inlet to the trap and a discharge outlet from the trap. When steam is first turned on, the disc is raised by the pressure and air in the line is discharged. Following condensate, which is cold at first, is also discharged. As the condensate temperature and pressure rise, flash steam is formed under the disc and the velocity of this passing below the disc towards the outlet increases, lowering the pressure under the disc so that the disc is drawn towards the seat rings. At the extreme circumference of the disc the velocity is less and there is a pressure build-up in a chamber above the disc until, at a point when the condensate temperature has approached that of steam, the flash-produced pressure in the chamber above the disc, acting on the large overall area of the disc, overcomes the inlet pressure (which acts on a smaller area of the disc) and the disc snaps shut against the seat rings to prevent further flow. Deprived of further flash steam, the pressure in the chamber above the disc falls, the inlet pressure asserts itself and the valve opens for the cycle to be repeated. However, if considerable amounts of air are encountered, so-called "air-binding" may occur, that is the trap may become held closed for long periods by air caught in the chamber above the disc. This phenomenon is discussed in some detail in British Patent Specification No. 1,178,160.